Newer network configurations address the need for higher density deployments and improved signal coverage, which in turn are needed for the higher data rate services demanded by subscribers. “Heterogeneous” networks stand as one such example. In a known heterogeneous network configuration, a “macro” base station provides coverage in a “macro” cell, while one or more associated “pico” base stations provide coverage in respective “pico” cells overlaying the macro cell. The term “pico” is generic as used here, and is meant to connote essentially any small, low-power network node, including “femto” base stations, “micro” base stations, residential access points, etc.
While it is known to operate macro and pico cells as separate cells within the larger network, where each such cell has its own cell ID, it is also known to use the same cell ID for a given macro cell and its one or more overlaid pico cells. In such “distributed” or “soft” cell deployment scenarios, each base station operating with the same cell ID can be understood as representing a different transmission point within the distributed cell formed by the associated macro and pico base stations. Further, in at least some implementations, the macro cell broadcasts system information, whereas most or all user-specific transmissions are limited to corresponding ones of the pico cells.
However, distributed cells and other heterogeneous network deployments offer significant opportunities for radio resource reuse. In an example case, multiple pico base stations within a distributed cell reuse the same control channel radio resources, at least where isolation between transmissions from the different nodes is sufficient. Here, “radio resources” connote particular times and/or particular frequencies, for example. Of course, the nature of the available resources will depend on the nature of the radio carrier and its approach to multiplexing and channelization.
The use of user-specific demodulation reference signals, “DMRS”, represents one technique for reducing interference between different control channel transmissions that share or otherwise reuse the same control channel frequency resources. For example, the use of DMRS that are configurable on a per “UE” basis for enhanced Physical Downlink Control Channel, “ePDCCH”, transmissions is under consideration by the Third Generation Partnership Project, “3GPPP”, for the Long Term Evolution, “LTE”, Advanced standard. Here, “UE” denotes an item of user equipment, and “UEs” denotes multiple items of user equipment.
An ePDCCH can be transmitted in two modes, either in a localized or a distributed manner. In distributed mode, the ePDCCH is divided into parts that are distributed over all N Physical Resource Block, “PRB”, pairs that have been configured for the UE, for ePDCCH reception. These N resources are generally widely spaced in frequency so that frequency diversity is achieved for the ePDCCH transmission. In localized mode, the ePDCCH is instead transmitted in one or, in case the ePDCCH does not fit into one PRB pair, a few of the N PRB pairs. The PRB pair or pairs can be selected by the eNB to achieve frequency selective scheduling gain, provided that the eNB has knowledge of which of the N PRB pairs have beneficial channel gain.
The configuration of per UE DMRS for ePDCCH transmissions would be similar to that already done for Physical Downlink Shared Channel, “PDSCH”, transmissions. The use of per UE DMRS for ePDCCH transmissions will facilitate spatial reuse of frequency resources in distributed cell deployment, such as where system information is broadcasted over a larger area than what is covered by some ePDCCH transmissions—see deployment “Scenario 4” in 3GPP TR 36.819, where multiple low power nodes have the same cell-ID as a macro node. In particular, using UE-specific DMRS for ePDCCH transmissions would reduce interference between control channel transmissions from different nodes that reuse some or all of the same resources and thus facilitate such reuse, assuming sufficient isolation exits.
In order to support frequency reuse between nodes, the DMRS sequence for a given UE needs to be reconfigured as the UE moves within the cell coverage and is connected to different ones of the base stations. The network accomplishes the required reconfigurations by sending a UE-specific Radio Resource Control, “RRC”, configuration message to the UE for each such reconfiguration. Example control channels subject to reconfiguration in the LTE Advanced context include the ePDCCH, the enhanced Physical Hybrid-ARQ Indicator Channel, “ePHICH”, the enhanced Physical Broadcast Channel, “ePBCH”, and the enhanced Physical Control Format Indicator Channel, “ePCFICH”.
Before adopting a new control channel configuration, the targeted UE must successfully receive and process the associated configuration message, which may change or update certain control channel configuration items, such as the time-frequency region used for the enhanced Control Channel, “eCCH”, the start OFDM symbol in the subframe, the search space, the set of used aggregation levels for blind decoding, the use of QPSK or 16 QAM modulation, the use of either localized or distributed transmission mode of the ePDCCH, and the DMRS sequence used for the eCCH.
Because the configuration message may not be successfully received and decoded by the targeted UE, the network cannot be sure that a given UE has applied a new control channel configuration until it receives a confirmation message from the UE, or until some other timeout applies. Thus, there exists a time of ambiguity between sending a configuration message to a UE that indicates a new downlink control channel configuration for the UE, and receiving confirmation from the UE that the new configuration has been applied. During this period or window of ambiguity, the network does not know which control channel configuration to use for sending a control message to the UE. That is, the base station that sent the configuration message does not know whether the UE is using the old control channel configuration that was in use when the configuration message was sent, or is using the new control channel configuration that was indicated by the configuration message.
Consequently, the conventional approach is to defer sending new control messages until the ambiguity is resolved. However, it is recognized herein that that approach has a number of disadvantages, including scheduling delays and possible service interruptions, leading to poorer user experience.